Fuse

ABSTRACT

The fuse comprises an elongated support formed at least partially from ferrite, magnetized in the direction of its thickness and on which is disposed, between two terminals, a metal wire (9) capable of being destroyed by melting. The fuse comprises for example two ferrite strips (5 and 6) assembled together and spaced apart by means of two insulating-material bars (7 and 8), a silver wire (9) being disposed between the two strips. When the wire melts, the molten metal and the electric arc are driven laterally in accordance with the LAPLACE law.

The present invention relates to a fuse comprising a metal wire capableof being destroyed by melting more especially in the case of an excesscurrent.

Fuses are devices, used already for a very long time, for protectingagainst excess currents. When the installation or the apparatus to beprotected is capable of withstanding an excess current for a short time,such fuses constitute a reliable protection. With the use of electroniccircuits, whose components are incapable of withstanding highovercurrents, even for very short times, being more and more widespread,it has proved that such fuses are not always capable of ensuring asufficient protection for the circuits. This is the case for example forremote data processing circuits connected to transmission lines exposedto overvoltages atmospheric in origin. The best fuses known up to datefor protecting such installations are silver-wire fuses enclosed in aglass tube, the wire being possibly stretched by means of a spring so asto ensure immediate breaking of the arc which forms at the moment whenthe wire breaks. Now even with the best fuses known up to date, thedestruction of circuit components can be noted despite the melting ofthe fuse. This is explained by the vaporization of the molten silverwhich forms a conducting plasma inside the tube, this plasma maintainingthe electric arc, i.e. a high current through the fuse. Generally, thiseffect may be seen by the blackening of the glass tube.

To ensure blowout of the arc, the use of deflection effects related tothe presence of a magnetic field has been proposed in some fuses, thismagnetic field being created by permanent magnets or coils associatedwith the fuse. Thus complex, expensive and bulky structures have beenprovided in the prior art. More especially, the metal permanent magnetsused in the constructions of fuses of the prior art require, on the onehand, the interpositioning of insulating walls between the magnet andthe fuse and, on the other hand, the use of bulky magnets. The spaceoccupancy of these magnets, due in particular to the fact that they arenecessarily magnetized in the direction of their length, practicallyrules out the formation of a uniform magnetic field over the wholelength of a fuse wire. The interpositioning of insulating wallsnecessarily increases the air gap or the distance between the pole pieceand the fuse, so that the magnetic field is reduced as well as theefficiency of the blowout.

The present invention has as its aim to ensure, by the simplest meanspossible, the extinction of this arc by blowout. According to one aspectof the invention, the means serving as permanent magnet also serve asmechanical support and as extinction chamber wall. According to anotheraspect of the invention, the means serving as permanent magnet aremagnetized in the direction of their thickness so as to reduce theirspace occupancy. According to other aspects of the invention, themagnetic field is produced over the whole length of the fuse wire so asto increase the mechanical stress produced in the wire and ensureblowout of the arc whatever the breakage point of the wire; the magneticfield is produced over a sufficient width on each side of the fuse wireto allow a substantially elongation of the arc and acceleration ofblowout.

In its simplest form, the fuse of the invention is formed by anelongated ferrite support magnetized in the direction of its thicknessand on which is disposed a non-ferromagnetic conductorfuse wire betweentwo terminals. According to the LAPLACE law, when the wire has a currentpassing therethrough, in one direction or in the other, it is subjectedto a force perpendicular to the wire and parallel to the ferrite supportplane and this force is proportional to the product of the current andof the magnetic field. It should be noted that this force acts on anymoving eletric charge, i.e. also on the electric arc likely to form atthe breakage point of the wire. This force not only results inmagnetically blowing out the electric arc, but in accelerating thebreakage of the wire at the point thereof weakened by melting and inaccelerating the separation of the strands at the breakage point, i.e.reducing the time during which an arc is likely to form.

According to a practical embodiment, the fuse is formed of two strips offlexible ferrite, formed from ferrite powder bonded by means of anelastomer, fixed at a small distance from one another by means of twoinsulating bars, the fuse wire being housed between the two ferritestrips. The electric insulating qualities of ferrites allow a very thinair gap to be formed in which the magnectic field is high. Furthermore,the effect of transverse blowout of the arc prevents the projection ofconducting material on the nearby ferrite walls, which projections wouldtend to prolong the existence of the arc.

The part of the insulating bar situated between the ferrite strips ispreferably provided with teeth, in the manner of a comb, so as to formgaps forming cooling and transverse extinction chambers in which the arcis broken up into fragments and magnetically blown out. If it is desiredto avoid the projection of metal outwardly through these holes, theselatter may be covered by means of an adhesive strip. The use of multiplecooling and transverse extinction chambers in relationship with themagnetic blowout allows a considerable extension of the path of the arcfor a given transverse dimension of the device, whereas the multiplecooling chambers of the prior art, not associated with magnetic blowout,only allow limited breaking up into fragments and expansion of themolten material. Thus a very rapid fuse is obtained with very highcut-off power.

According to another aspect of the invention, the teeth forming thetransverse walls of the cooling chambers also serve as an intermediatemechanical support for the fuse wire, the wire being nipped between theopposite teeth.

The terminals may be formed in different ways, for example by means ofrings or by magnetized plugs nipping the ends of the wire.

The enclosed drawing shows, by way of example, a few embodiments of theinvention.

FIG. 1 is a perspective view of a first, and simplest, embodiment of theinvention.

FIG. 2 is a partial perspective view of a second embodiment, having twomagnetized strips.

FIG. 3 shows one embodiment of the terminals.

FIG. 4 shows another embodiment of the terminals.

FIG. 5 is a top view of a third embodiment, one of the ferrite stripsbeing removed.

FIG. 6 is a perspective view of this third embodiment.

FIG. 7 illustrates one method of forming the terminals in the thirdembodiment.

FIG. 8 shows a variation, with armature, of the third embodiment.

FIG. 1 illustrates simultaneously the principle of the invention and thesimplest embodiment thereof. On a thin ferrite slab 1 magnetized in thedirection of its thickness so as to present north poles on its lowerface and south poles on its upper face, is fixed a silver wire 2 bymeans of two metal rings 3 and 4 which form the terminals of the fuse.Wire 2 has, for example, a diameter of 0.1 mm and a length of 30 mm.According to the LAPLACE law, when this wire has a current I passingtherethrough, it is subjected to a force F under the effect of themagnetic field H.

According to the embodiment shown in FIG. 2, the fuse is formed from twoflexible strips 5 and 6 formed from ferrite powder bonded by means of anelastomer, which is commercialized under the name PLASTOFERRITE. Thesestrips are magnetized in the direction of their thickness and attracteach other mutually. They are fixed face to face and maintained apartfrom each other in the direction of their thickness by means of twoT-section insulating bars 7 and 8. Strip 5 is fixed to bars 7 and 8 bybonding, whereas strip 6 is simply held magnetically by strip 5 so as toremain removable. The legs 70 and 80 of the T-section ensure the spacingof the magnetized strips, which spacing defines the thickness of ahousing 90 in which is disposed the silver wire 9. The transversedimension of housing 90 is defined by the spacing of legs 70 and 80. Atits ends, the wire 9 is fixed and connected galvanically to twoterminals which may be formed as shown in FIG. 3 in which each terminalis formed by a metal strap 10 whose ends 10a and 10b are bent backaround each of the insulating bars 7 and 8, the end of wire 9 being bentback under the strap. The end of wire 9 could also be welded to thestrap. The upper magnetized strip allows easy reloading of the fuse.

In the embodiment shown in FIG. 4, the ends of wire 9 are fixed bynipping between two magnetized metal pieces 11 and 12 formingsimutaneously plugs closing the ends of the fuse and contact terminals.Referring to FIG. 2, it will also be possible to nip wire 9 between twometal plates bonded respectively to the upper face of strip 5 and to thelower face of strip 6. So as not to attenuate the magnetic fieldnon-ferromagnetic, preferably diamagnetic metals will be used. Theferrie strips may have for example a length of 50 mm for a width of 10mm and a thickness of 1.8 mm with an air gap of 1.5 mm.

It is possible to increase the efficiency of the effect of magneticblowout of the electric arc by using insulating bars 13 and 14 such asshown in FIGS. 5 and 6. The legs of these insulating bars 13 and 14 areprovided with teeth 15 and 16 engaging between the ferrite strips 6 and7, these teeth forming two combs whose teeth are situated opposite eachother, the gaps 17 and 18 formed between the teeth constituting arccooling and extinction chambers. The legs and the teeth of bars 13 and14 have a sufficient length for the fuse wire 9 to be held moreovermechanically between these teeth, which allows it to withstand morereadily mechanical shocks without risk of breaking. It has in fact beennoted that fuse wires of the prior art break by simple mechanical stressat their fixing point. It is moreover possible to cause the extinctionchambers to communciate with the outside through holes 19 which extendthem and which further promote the blowout of the arc and the expulsionof metal particles. If necesary, these holes may be closed by means ofadhesive strips 20 and 21 shown with broken lines. At its ends, wire 9is nipped between two wider teeth 22 and 23 of the insulating bars. Theends of the wire may be connected to terminals by nipping or soldering.FIG. 7 illustrates one example of connection in which the wire is bentback into a slit 24 in one of the bars and under a metal ring 25.

It is possible to considerably increase the magnetic field, i.e. theblowing-out effect in the air gap, by means of a ferromagnetic armature.Such an embodiment is shown in FIG. 8, as a variation of the embodimentshown in FIGS. 5 and 6, in which the armature is formed by a soft-ironmetal sheet 26 bent into a U. With this armature, it was possible tomeasure a magnetic field of 1200 to 1300 Gauss against a field of 500 to650 Gauss without armature. The forces acting in this case on the wireand the arc are relatively very high.

To sum up, the fuse of the invention presents a very high cut-off powerand ensures a practically absolute protection for delicate components byits high cut-off speed, it is easy to manufacture, it may be reloadableand even cleanable.

The present invention is not limited to the embodiments which have beenexplicitly described, but includes the different variations andgeneralizations thereof contained within the scope of the followingclaims.

I claim:
 1. A fuse comprising a conducting wire (9) capable of beingdestroyed by melting in the case of an excess current; at least oneelongated support means formed at least partially of ferrite (1) andhaving a top surface, a bottom surface and a thickness between said topand bottom surfaces, magnetized in the direction of its thickness and onwhich said conducting wire is disposed between two terminals (3, 4)spaced apart along said support means, said wire being supported alongits entire length on one of said surfaces of said support means.
 2. Thefuse of claim 1, comprising two ferrite support means (5, 6) assembledand spaced apart by means of two insulating material bars (7, 8) eachferrite support means being formed as an elongated member having a topsurface, a bottom surface and a thickness therebetween, each saidsupport means being magnetized in the direction of its thickness, saidwire being located between said support means.
 3. The fuse of claim 2,wherein the ferrite strips (5, 6) are flexible strips.
 4. The fuse ofclaim 3, wherein one of the ferrite strips (5) is fixed by bonding tothe insulating bars whereas the other (6) is held magnetically byattraction by means of the bonded strip (5).
 5. The fuse of claim 2,wherein portions (70, 80) of the insulating bars situated between theferrite strips (5, 6) are provided with teeth (15, 16) defining gaps(17, 18) forming electric-arc extinction chambers.
 6. The fuse of claim5, wherein said conducting wire is nipped between the teeth (15, 16) ofthe insulating bars.
 7. The fuse of claim 5, wherein the bottom of thegaps (17, 18) are provided with a hole (19) situated in the extension ofthe gap promoting blowout of the arc and expulsion of metal particles.8. The fuse of claim 2, wherein the ferrite strips (5, 6) are wrapped ina ferromagnetic armature (26).